PITTSBURGH—Too much garlic mustard in your neighborhood forest? Actually, the problem may be too many deer.

A research team led by Susan Kalisz, professor of evolutionary ecology in the University of Pittsburgh’s Department of Biological Sciences, published a paper online today in the Proceedings of the National Academy of Sciences that takes a long view on why invasive garlic mustard plants thrive to the detriment of native species.

The study, initiated in 2003 at the Trillium Trail Nature Reserve in Fox Chapel, Pa., concludes that an overpopulation of deer (density of deer in the United States is about four to 10 times what it was prior to European settlement of North America) is the primary reason garlic mustard is crowding out native plants, such as trillium, which are preferred food for wild deer.

Garlic mustard (Alliaria petiolata) is a plant native to Europe and Asia and is inedible by deer standards. It was brought to the United States—Long Island, N.Y., specifically—in the 1860s for use as a kitchen herb.

Instead it became a menace, colonizing forest floors in the Eastern United States and Canada and has been found in Washington, Utah, and British Columbia, achieving the dubious distinction of being one of very few non-native plants to successfully invade forest understories. The persistence of garlic mustard greatly reduces forest biodiversity.

To study the effect of rampant deer on trillium and garlic mustard populations, Kalisz and colleagues established multiple 196-square-meter plots in the forest. Half were fenced to exclude deer. Years of observation and hours of statistical analysis later, Kalisz and her colleagues have found that in plots where deer were excluded, the trillium population is increasing and the garlic mustard population is trending toward zero.

“This demonstrates that the high population growth rate of the invader is caused by the high abundance of deer,” she says. This effect is reversible with deer exclusion. The team’s results support “an ecological theory that native species in a community can exert biotic resistance.” This means that native plants as a group can successfully compete against invaders. If the native plants are allowed to thrive rather than being consumed by deer, the combined natural competitive advantages of those plants—including trillium—allow them to repel the outsiders.

This study shows that two major ecological and management problems are linked: an excess of deer in temperate forests and an invasion of these forests by exotic plants. Similar links may be found in other ecosystems between overabundant native or managed fauna (such as cattle or sheep) and declining diversity of flora. Management of overabundant animals could be beneficial for conservation of plant biodiversity in general.

“When people walk in the woods where deer are overabundant, they don’t realize what’s missing,” Kalisz says. “They don’t know what used to be growing there. They don’t know that species are being lost and replaced by invaders.”

The solution seems simple, then: Reduce deer populations, restore natives, and prevent invasion. It’s not simple, Kalisz says. Deer management policies vary from state to state, and deer don’t respect political boundaries. Some states keep deer populations low, while others prefer to maintain higher populations to appeal to groups such as hunters. Yet, deer, Kalisz says, exact a toll not only on forest species but also on farms, orchards, and even your car and your car insurance rate.

Kalisz notes that at the very moment she submitted her manuscript to PNAS, she saw a six-point buck ambling down the street in front of her home in Pittsburgh’s East End.

Kalisz’s coauthors are Carol Horvitz with the University of Miami’s Department of Biology and Rachel Spigler, a former Pitt postdoctoral associate with Kalisz, now a faculty member with Temple University’s Department of Biology. The work at Trillium Trail was funded by a grant to Kalisz by the National Science Foundation and support from Fox Chapel Borough. The paper, “In a long-term experimental demography study, excluding ungulates reversed invader’s explosive population growth rate and restored natives,” can be found here.